Determination of moisture (or volatile) content in materials is of such importance in so many fields that a wide variety of devices and analytical methods are used. One method for measuring moisture content in solids and non-volatile liquids is thermal gravimetric. In a thermal gravimetric system, a small sample of material is weighed and then the material is dried by the application of heat thereto and re-weighed after drying. Any difference in weight is indicative of the moisture content.
In one conventional thermal gravimetric moisture analyzer, the sample material is placed on a sample holder which is attached to a high precision scale, and enclosed by an electrically heated chamber. The temperature of the air in the chamber is measured and controlled to a selected value. The selected value is the temperature at which the volatiles in the sample material should be successfully driven off without allowing the sample material to burn. It is assumed that the temperature of the sample material correlates to the air temperature, hence when the air temperature is at the selected value, it is presumed that the sample material is also at the selected value. Following the heating process, the resulting loss of sample weight is automatically determined and displayed.
The electrical heater of the prior art moisture analyzer generates heat energy that has a convective component and a radiative component. The convective component is the heat energy that the sample receives from the movement of the warmed air in the chamber. The radiative component is the heat energy that the sample receives by absorption of infrared radiation emitted from the electrical heater. A problem with this conventional electrical heater is that air does not receive a significant amount of heat energy by absorption of infrared radiation. Thus, the air temperature measurement is primarily a measure of the convective component of the heat energy, not a combined measure of both the convective and radiative components of the heat energy such as that imparted on the sample material. Hence, the air temperature measurement is not an accurate representation of the actual sample temperature.
Furthermore, heating elements of different moisture analyzers have varying levels of radiative emissions. Thus, the radiative component of heat energy may not be consistent between heaters and even from one test time to the next. In addition, radiative absorptivity of the sample material varies from material to material. The radiative absorptivity is the amount of heat that a sample material absorbs from the radiative component of the heat energy generated by the moisture analyzer. Due to these above variations, the sample material temperature cannot be reliably correlated to the measured air temperature.
Unreliable correlation leads to variations in the amount of volatiles lost by the sample material and the rate at which the volatiles are driven off from test to test and from system to system. For example, when the air temperature reaches the selected value, the radiative component of the heat energy may cause the sample temperature to be higher than the air temperature, thus the sample material may undesirably burn and lose hydrocarbons as well as volatiles so that the weight loss is inaccurately high. Whereas, during a different test of the same type of sample material, the radiative component of the heat energy differ from the previous test so that the results from the previous test cannot be correlated with the later test to prevent the excess heating of the sample material and the resulting loss of hydrocarbons.
To circumvent the problems associated with the variability of the radiative component of the heat generated by the electrical heater, other prior art gravimetric moisture analyzer systems utilize a microwave oven to dry a sample material. The microwave heated moisture analyzer solves the problems associated with the electrically heated moisture analyzer because there are no convective and radiative components of heat energy such as that generated by an electrically heated moisture analyzer. As is known in the art, microwave radiation is absorbed by water and polar organic molecules, causing an increase in the molecular motion. Due to the absorption of radiation energy, the water and polar solvents are collectively heated and removed through vaporization and volatilization.
Air temperature inside the microwave oven is not used to control the microwave oven to a preselected value, since microwave heating does not involve the convection of heat energy. Rather, the microwave oven is configured to dry the sample material for a preselected time. The drying time is preselected, based on factors such as the sample size, the magnetron capabilities of the microwave, expected time to achieve volatile loss, and other factors which may be determined by experimentation.
A problem with the microwave heated moisture analyzer is that if the sample is heated for too long of a duration, the microwave radiation may produce hot spots in the sample material which could decompose or destroy part of the sample being tested. Furthermore, since the preselected drying time is determined by experimentation, drying time does not automatically adapt to individual variability of the sample material, such as a higher than normal moisture level in the sample material, unexpected volatile loss, variability of sample size, and so forth. Thus, use of the microwave heated moisture analyzer also introduces inaccuracies into the data.
To gain more accurate and reliable drying of the sample material, it is desirable to directly measure the temperature of the sample material. Unfortunately, in either of the prior art moisture analyzers, it is not possible to directly contact the sample material to obtain a measure of the sample temperature because such direct contact may interfere with the high precision weighing process.